The present invention relates to calculating apparatus and, in particular, to calculating apparatus for calculating the roof square sum of a number of values.
The root square sum of a number of values is required to be calculated in a variety of circumstances. The apparatus for effecting the calculation should be capable of being manufactured relatively easily at low cost whilst providing the required degree of accuracy. Such a calculation is needed, for example, in magnetic measurement when it is required to determine the absolute value of the strength of a magnetic field. The strength of a magnetic field is required in, for example, geometric surveys, detection of ferrous metals or in the evaluation of the field produced by electrical equipment.
For measurement of magnetic fields a sensor is used which comprises of three single axis orthogonally mounted magnetometers. The magnetometers are accurately calibrated to eliminate scaling factors, offset and alignment errors and are, for accurate measurement, of the `Second-Harmonic Fluxgate` type that yield excellent stability and linearity over a wide dynamic range.
The output signals provided by the magnetometers correspond to orthogonal components of the surrounding magnetic field and the value of the `total magnetic field` is the root-sum-square of these three components, i.e. the vector sum of the components.
Typically, known circuits for the calculation of the root-sum-square comprises a configuration of analogue multipliers. However, the use of such multipliers has several disadvantages. For example, analogue multipliers of the required performance are relatively expensive and each requires external compensation to account for offsets and gain mismatch. Furthermore, the nature of the algorithm requires a large dynamic range of each multiplier used to square the respective components. In particular, this requirement can result in large errors when the respective field components are small as low level input signals are received by the multipliers which are amplified when the square root function is carried out, the square roof of a small number being a larger number. Additionally, to resolve three magnetic field components requires the use of four analogue multipliers, a high performance operational amplifier, and a large number of discreet passive components. These add to the cost and complexity of the circuit and give rise to a relatively high power consumption level, typically 30 mA for known circuits, which is unacceptable in many applications where it may be required to monitor the total magnetic field over a prolonged period.